1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a method of writing to a magnetic random access memory device.
2. Description of the Related Art
A magnetic random access memory (MRAM) is a non volatile memory device which utilizes a magnetoresistance effect to store information. In the magnetoresistance effect, resistance of an electrically conductive material is changed based on a circumferential magnetic field. An MRAM device includes a plurality of MRAM cells composed of magnetic tunnel junctions (MTJs) on a single transistor.
The MTJ is composed of multi thin layers such that electrons may cause tunneling through a very thin insulating layer sandwiched between two ferromagnetic electrodes when an external electrical signal is applied thereto. The top electrode of the two electrodes is called a free layer and the bottom electrode is called a pinned layer.
When magnetization directions within the free layer and the pinned layer are arranged parallel to each other, a tunneling current flowing through the MTJ has its maximum value. That is, the tunneling resistance has its minimum value. In contrast, when the magnetization directions within the free layer and the pinned layer are arranged anti-parallel to each other, the tunneling current flowing through the MTJ has its minimum value. That is, the tunneling resistance has its maximum value.
The MRAM utilizes magnetization to store information while the conventional memory utilizes electric charge. That is, digital data represented as ‘0’ and ‘1’ are stored differently based on the low resistance state where the magnetization directions of the two electrodes are parallel to each other and the high resistance state where they are anti-parallel to each other.
An anti-ferromagnetic layer which is referred to as a pinning layer is added to the pinned layer. The pinning layer acts to fix the magnetization direction of the pinned layer. That is, the pinned layer attached to the pinning layer has a large switching field, and the magnetization direction of the pinned layer is fixed always in the constant direction when an applied magnetic field is smaller than the switching field. Thus, data of the MRAM cell may be determined based on the magnetization direction within the free layer. The magnetization direction of the free layer may be changed by applying a magnetic field to its circumference or perimeter. In order to change the magnetization direction of the free layer to a desired direction, conductive layers such as a bit line and a digit line are formed to be orthogonal to each other above and below the MTJ. Current flows through each conductive layer, so that a magnetic field is generated therefrom to be used for changing the magnetization direction.
In this case, when the magnetization direction of the MTJ selected to store data is changed, a magnetization direction of the MTJ that is not selected should not be changed. However, in order to enhance the integration density within a limited space, not only the size of the MTJ but also the spacing between the MTJs should be decreased. Due to the decreased spacing between the MTJs, the magnetic field that has been generated to change the magnetization direction of the selected MTJ increasingly affects adjacent MTJs that were not selected. When this effect becomes severe enough to invert the magnetization direction of the adjacent MTJs, there cannot be a normal data storage operation.
To cope with the above-mentioned problems, another writing method referred to as toggle switching has been proposed. A writing method for an MRAM device employing the toggle switching is disclosed in U.S. Pat. No. 6,545,906 B1 entitled “Method of writing to scalable magnetoresistance random access memory element” to Savtchenko et al.
According to U.S. Pat. No. 6,545,906 B1, a digit line is positioned on a predetermined region of a semiconductor substrate. A word line is positioned above the digit line and crosses the digit line. In this case, the word line is substantially the same as a bit line. An MTJ is interposed at an interconnection between the digit line and the bit line in a tilted direction of 45° from the intersection. The MTJ includes a second magnetic region, a tunneling barrier, and a first magnetic region which are sequentially stacked. Each of the first and second magnetic regions has a synthetic anti-ferromagnetic (SAF) structure. The SAF structure is composed of a top ferromagnetic layer, a bottom ferromagnetic layer, and an anti-ferromagnetic coupling spacer layer interposed therebetween.
FIG. 1 shows a switching characteristic of an MRAM cell fabricated in accordance with U.S. Pat. No. 6,545,906 B1.
Referring to FIG. 1, the horizontal axis indicates a magnetic field (HW) induced to the word line, in units of oersteds (Oe), which is the unit of magnetoresistance and is proportional to the magnitude of the word line current. The longitudinal axis indicates a magnetic field (HD) induced to the digit line, also in units of oersteds (Oe) which is proportional to the magnitude of the digit line current.
There are three regions in the switching characteristic diagram, i.e., a no switching region 2, a direct switching region 5, and a toggle switching region 7. The toggle switching region 7 has a large switching area as shown in FIG. 1.
FIG. 2 shows a current waveform applied when writing operations of MRAM devices fabricated in accordance with U.S. Pat. No. 6,545,906 B1 are performed.
Referring to FIG. 2, the horizontal axis in the current waveform view denotes a lapse of time. The waveform of FIG. 2 indicates that a word line positive current pulse IW is applied to the word line 11 during the time from t1 to t3 and a digit line positive current pulse ID is applied to the digit line 12 during the time from t2 to t4 while the writing operations are performed during the time from t0 to t4.
The writing method utilizing the toggle switching region 7 begins with reading of an initial state of the MTJ. For example, in the case that the initial state of the MTJ is read as ‘1’, when the word line positive current pulse IW is applied to the word line 11 during the time from t1 to t3 and the digit line positive current pulse ID is applied to the digit line 12 during the time from t2 to t4, the magnetization direction of the MTJ is changed to write a ‘0’ state. Subsequently, when the word line positive current pulse IW is applied again to the word line 11 during the time from t1 to t3 and the digit line positive current pulse ID is applied again to the digit line 12 during the time from t2 to t4, the magnetization direction of the MTJ is changed again to write a ‘1’ state. In this case, the word line positive current pulse IW and the digit line positive current pulse ID are applied with a time delay as shown in the figure, however these pluses constitute a sequence which has an overlapped region therebetween. In addition, the magnitude of the current pulse corresponds to the toggle switching region 7.
FIG. 3 is a partial plan view illustrating a portion of an MRAM device and a method of the prior art employing toggle switching.
Referring to FIG. 3, according to the MRAM device using toggle switching, digit lines DL are disposed on a predetermined region of a semiconductor substrate, and word lines WL crossing the digit lines DL are disposed on the digit lines DL. MTJs 16 are interposed at intersections between the digit lines DL and the word lines WL in a tilted direction of 45°. Each of the MTJs has a bottom electrode 14.
In general, the MTJ 16 has a magnetization easy axis and a magnetization hard axis. The magnetization easy axis is formed in a direction having a geometric maximum length of the MTJ 16, and the magnetization hard axis is formed in a direction orthogonal to the magnetization easy axis. Accordingly, by making the geometric shape of the MTJ 16 elliptical or rectangular or performing annealing with a magnetic field being applied, the direction of the magnetization easy axis is adjusted.
To use toggle switching, the magnetization easy axis of the MTJ 16 should be disposed in a tilted direction to be oriented at an angle of about 45° to the digit line DL or the word line WL. In this case, the digit line DL and the word line WL should be orthogonal to each other, and the MTJ 16 should be disposed at the intersection therebetween to be oriented at an angle of about 45°. As a result, it is difficult to dispose a plurality of MTJs 16 in a limited area. That is, it is not efficient to implement the MRAM device in this configuration with a high integration density.